1. Field of the Invention
The present invention relates to a method of irradiating semiconductor films using laser light, and to a laser irradiation apparatus (apparatus containing a laser and an optical system for introducing laser light output from the laser to an irradiation subject) for performing irradiation of semiconductor films.
2. Description of the Related Art
Techniques for increasing crystallinity or performing crystallization by irradiating laser light to a semiconductor film formed on an insulating substrate such as glass have been widely researched in recent years. Silicon is often used in the semiconductor film. Means of crystallizing a semiconductor film by using laser light and obtaining a crystalline semiconductor film, is referred to as laser crystallization throughout this specification.
Compared to conventional synthetic quartz glass substrates that are in widespread use, glass substrates possess the advantages of having abundant workability at low cost, and of easily manufacturing a large surface area substrate. These are the reasons the aforementioned research is being carried out. Further, the use of lasers, preferably for crystallization, is due to the low melting point of glass substrates. Lasers are able to impart a high amount of energy to the semiconductor film only, without increasing the temperature of the substrate. Further, throughput is remarkably high in comparison with means of heat treatment using an electric furnace.
Crystalline semiconductors are made up of many crystal grains, and therefore are also referred to as polycrystalline semiconductor films. Crystalline semiconductor films formed by irradiating laser light, have high mobility, and therefore thin film transistors (TFTs) are formed using crystalline semiconductor films. For example, crystalline semiconductor films are utilized much in devices such as monolithic liquid crystal electro-optical devices in which pixel driver TFTs and driver circuit TFTs are manufactured on one glass substrate.
Further, a method in which pulse laser light such as an excimer laser having a high output is optically processed into a square spot of several centimeters per side, or into a linear shape having a length equal to or greater than 10 cm, and the laser light is then scanned (alternatively, the position of laser light irradiation is moved relative to the irradiation surface) and irradiated onto the surface, is good for mass production and is industrially superior. This method is therefore preferably used.
In particular, if a linear shape beam is used, laser irradiation can be performed on the entire irradiation subject by only scanning in a direction perpendicular to the longitudinal direction of the linear shape beam, differing from the case of using spot shape laser light in which forward and backward, and left and right scanning is necessary. Mass production is therefore good. The reason for scanning in a direction perpendicular to the longitudinal direction is because the scanning direction has the highest efficiency. In present methods of laser irradiation, the use of linear shape beams, in which pulse emission excimer laser light is processed by a suitable optical system, is gaining ground as a technique for manufacturing liquid crystal display devices using TFTs, due to its good mass production characteristics.
Semiconductor film crystallization after irradiating laser light to a semiconductor film is explained here. If laser light is irradiated to a semiconductor film, the semiconductor film will melt. However, the temperature of the semiconductor film drops as time passes, and crystal nuclei form. An almost countless number of uniform (or non-uniform) crystal nuclei are generated in the semiconductor film, and crystallization is complete after they nuclei grow. The position and size of the crystal grains obtained in this case, are random. Further, the crystal grain growth distance is known to be proportional to the product of the crystallization time and the growth speed. Here, the term crystallization time is the amount of time from when the crystal nuclei develop within the semiconductor film until crystallization of the semiconductor film is complete. If the amount of time from the melting of the semiconductor film until crystallization is complete is taken as melting time, the melting time increased, and the cooling speed of the semiconductor film is taken as being leisurely, then the crystallization time becomes long, and crystal grains having a large grain size can be formed.
There are several different types of laser light, but in general, laser crystallization utilizing laser light having a pulse emission excimer laser (hereafter referred to as excimer laser light) is used. Excimer lasers have the advantages of high output, and the capability of repeated irradiation at high frequency, and in addition, excimer laser light has the advantage of a high absorption coefficient with respect to silicon films.
KrF (wavelength 248 nm) and XeCl (wavelength 308 nm) are used as excitation gases in excimer lasers. However, Kr (krypton) and Xe (xenon) gasses are extremely high cost, and if the frequency of gas replacement becomes high, this invites an increase in manufacturing costs.
Further, it is necessary to replace parts such as a laser tube for performing laser emission, and a gas purification apparatus for removing unnecessary compounds generated in the process of emission, on a 2 to 3 year basis. These attached parts are often expensive, and this also invites a problem of increased manufacturing costs.
Laser irradiation devices using excimer laser light possess high performance, as stated above, but require an extreme amount of efforts for maintenance. In addition, they also possess the disadvantage of high running cost when used as mass production laser irradiation devices (the term running cost meaning costs that develop along with operation).
In order to realize a laser irradiation apparatus having a low running cost compared with an excimer laser, and to realize a laser irradiation method using the laser irradiation apparatus, a method of using a solid state laser (a laser which outputs laser light with crystal rods as resonance cavities) can be used.
However, the grain size of crystal grains formed in accordance with laser crystallization using a YAG laser, which is one typical solid state laser, is extremely small compared to crystal grains formed by laser crystallization using an excimer laser.
It is thought that one reason is that although solid state lasers have high output at present, the output time is extremely short. Methods such as LD (laser diode) excitation and flash lamp excitation exist as methods of solid state laser excitation. In order to obtain high output by LD excitation, it is necessary to have a large electric current flow in LD. The LD lifetime is therefore short and the cost is increased compared with flash lamp excitation. For this reason, almost all LD excitation solid state lasers are small output. High output lasers for use in mass production are still in a development state at present. On the other hand, flash lamp can output an extremely strong light, and therefore lasers excited by flash lamps have high power. However, atoms excited by energy introduced instantaneously are emitted all at once with emission by flash lamp excitation, and therefore the laser output time is extremely short. Thus, solid state lasers at present have high output, but their output time is extremely short. Consequently, it is difficult to form crystal grains by laser crystallization using a solid state laser that have a grain size which is in the same order as, or greater than, the grain size formed by performing laser crystallization using an excimer laser. Note that the term output time refers to the half width of one pulse within this specification.
An object of the present invention is to provide a laser irradiation apparatus having low running cost in comparison with conventional laser irradiation apparatuses. In addition, an object of the present invention is to provide a laser irradiation apparatus for forming crystal grains having a grain size which is on the same order as, or is greater than, that of conventional crystal grains in a method of laser irradiation using the laser irradiation apparatus.
In order to form crystal grains having a grain size that is in the same order as, or is greater than, the grain size of crystal grains formed in accordance with laser crystallization using an excimer laser, first calculations are performed relating to temperature changes during irradiation of a semiconductor film by an excimer laser. Temperature versus time at points A to C in FIG. 3 was calculated for irradiation of excimer laser light to a silicon film made from the structure shown in FIG. 3. The output time of the laser light is taken as 27 ns here, and the energy density is set from 0.1 to 0.5 J. Results are shown in FIGS. 7A to 7G. It can be seen from FIGS. 7A to 7G that the crystallization time and the melting time become longer with increasing energy density, and that the cooling speed becomes slower. Further, it can be seen that a change in the temperature of the point A follows the temperature of point C.
Slowing down the cooling speed of the semiconductor film can be given as one effective means for forming large size crystal grains. Specifically, a method in which the laser light output time is made longer, and the semiconductor film melting time is also lengthened.
Calculations relating to temperature change when lengthening the output time of a YAG laser and performing irradiation to a semiconductor film were then performed. As shown in FIG. 3, laser light from a YAG laser is irradiated to a silicon film having a film thickness of 50 nm and formed on a silicon oxide film, and temperature versus time is calculated in the silicon film surface (the point A), in the interface between the silicon film and the silicon oxide film (the point B), and in the silicon oxide film at a distance of 100 nm below the interface (the point C). The temperature at which the silicon film melts is set at 1200 K here. Results are shown in FIGS. 4A through 6F. The output time was set to 6.7 ns and the energy density was set from 0.15 to 0.4 J in FIGS. 4A to 4D. In FIGS. 4E to 4H, the output time is set to 20 ns and the energy density was from 0.2 to 0.5 J. The output time was set to 27 ns for FIGS. 5A to 5D, and to 50 ns in FIGS. 5E to 5H, with the energy density varying form 0.2 to 0.5 J. In FIGS. 6A to 6C, the output time was set to 100 ns and the energy density was set form 0.3 to 0.5 J, while the output time was set to 200 ns in FIGS. 6D to 6F, with an energy density varying from 0.4 to 0.6 J.
The temperature of the points A to C increased due to irradiation of laser light, and after maintaining a first fixed temperature, there is an additional increase and a maximum temperature is achieved. It can be seen that the temperature of the points A to C then drops, and a second fixed temperature is maintained, and that there is a tendency to have an additional drop in temperature. The calculations were performed with the melting temperature of the silicon film taken as 1200 K, and therefore the silicon film is melted at the first fixed temperature, while solidification of the silicon film (crystallization) occurs at the second temperature. The time from the start of the second fixed temperature until the completion time corresponds to the crystallization time. The longer the crystallization time, the slower the cooling speed. Further, if the time from the beginning time of the first fixed temperature until the completion time of the second fixed temperature is taken as the melting time of the silicon film, the amount of time until the highest temperatures in the points A to C are achieved increases and the melting time becomes long with increasing output time at the same energy density. Namely, it can be said that the cooling speed of the semiconductor film becomes more relaxed with increasing output time.
Furthermore, the temperature of the silicon oxide film versus the laser light output time when crystallization begins is shown in FIG. 12. From FIG. 12 it can be seen that the temperature of the silicon oxide film at the beginning of crystallization increases with lengthening output time. In addition, the temperature of the silicon oxide film drops rapidly when the laser light output time is equal to or less than 50 ns. In other words, it is effective to increase the temperature of the base film in order to extend the amount of melting time for the semiconductor film.
The crystallization time and the melting time thus become longer, and the cooling speed of the semiconductor film thus becomes slower, with lengthening output time. The density of crystal nuclei generated becomes low, and the crystallization time becomes long; large size crystal grains can thus be formed. In other words, lengthening the output time is an effective means of making the crystal grains large.
However, as already discussed, solid state lasers at present have high output, but their output time is extremely short. For example, the output time of the model L4308 XeCl excimer laser (wavelength 308 nm) from Lambda Physic Corporation is 27 ns, while the output time of the DCR-3D Nd:YAG laser (wavelength 532 nm) of Spectra Physic Corporation is from 5 to 7 ns.
The present invention provides a laser irradiation apparatus, and a laser irradiation method, for forming crystal grains when irradiating a semiconductor film with laser light having a short output time using a solid state laser (a laser that outputs laser light using crystal rods as resonance cavities) as a light source, the grain size of which is in the same order as, or greater than, the grain size achieved for a case of irradiating a semiconductor film using laser light having a long output time. This result is achieved by forming a lag in other laser light, and irradiating the other laser light to the semiconductor film, making the cooling speed of the semiconductor film slower.
It is preferable that the laser light be formed into a linear shape by an optical system at this point. Note that the term formation of laser light into a linear shape refers to processing laser light such that it will have a form, which is linear on an irradiation surface. In other words, the cross sectional shape of the laser light is formed into a linear shape. Further, the term linear shape does not refer to the strict meaning of line, but refers to a rectangular shape (or elliptical shape) having a large aspect ratio. For example, this indicates an aspect ratio equal to or greater than 10 (preferably between 100 and 10,000).
In general, known solid state laser can be used as the solid state laser; lasers such as YAG lasers (normally indicating Nd:YAG lasers), Nd:YLF lasers, Nd:YVO4 lasers, Nd:YAlO3 lasers, ruby lasers, Ti:sapphire lasers, and glass lasers can be used. In particular, it is preferable to use a YAG laser, which has superior coherency and pulse energy.
Note that the fundamental harmonic (the first harmonic) has a long wavelength of 1064 nm, and therefore it is preferable to use the second harmonic (wavelength 532 nm), the third harmonic (wavelength 355 nm), or the fourth harmonic (wavelength 266 nm). These harmonics can be obtained by using nonlinear crystals.
The first harmonic can be modulated into the second harmonic, the third harmonic, or the fourth harmonic by using a wavelength modulator containing nonlinear elements. Formation of each harmonic may be performed according to known techniques. Further, the term laser light from a solid state laser as a light source includes not only the first harmonic, but also second harmonics, third harmonics, and fourth harmonics which wavelengths are modulated into.
Furthermore, a Q switch method (Q modulation switch method) often used by YAG lasers may also be used. This is a method for pulse laser output in which the energy value has an extremely precipitous rise due to suddenly increasing the Q value from a state in which the Q value of the laser resonance apparatus is sufficiently low. This is a known technique.
The solid state laser used by the present invention is basically capable of outputting laser light provided that a solid state crystal, a resonance mirror, and a light source for exciting the solid state crystal are present, and therefore there is very little maintenance time and effort compared to excimer lasers. Namely, the running cost is extremely low compared to that of an excimer laser, and therefore it becomes possible to greatly lower the manufacturing costs of semiconductor devices. Further, the availability ratio of a mass production line is increased if the amount of maintenance decreases, and therefore the overall throughput in the manufacturing process increases. This also greatly contributes to a reduction in the manufacturing costs of semiconductor deices. In addition, the surface area occupied by the solid state laser is small compared to that occupied by excimer lasers, and this is advantageous in design of the manufacturing line.
With the present invention, the cooling speed of a semiconductor film during laser crystallization using laser light having a short output time is made slower. This is accomplished by irradiating a plurality of laser lights in which time differences are formed. The amount of time allowed for crystal growth is increased by this crystallization process, and as a result, the size of the crystal grains formed becomes larger.